A radio-immunoassay (RIA) has been developed which can detect and identify malaria sporozoites in wild-caught mosquitoes. The presence, quantity and species of malaria parasite can be determined in dead mosquitoes stored for months at room temperature. This assay should become an important tool in investigations of malaria epidemiology in support of vector control programs or vaccine trials. Monkeys can be immunized with radiation-anttenuated sporozoites of Plasmodium knowlesi. Hybridoma produced monoclonal antibodies as well as serum from immunized animals have been used to identify a single protein (MW 42000) on the sporozoite surface which appears to act as the protective antigen.